Root/fs/jffs2/README.Locking

1
2    JFFS2 LOCKING DOCUMENTATION
3    ---------------------------
4
5At least theoretically, JFFS2 does not require the Big Kernel Lock
6(BKL), which was always helpfully obtained for it by Linux 2.4 VFS
7code. It has its own locking, as described below.
8
9This document attempts to describe the existing locking rules for
10JFFS2. It is not expected to remain perfectly up to date, but ought to
11be fairly close.
12
13
14    alloc_sem
15    ---------
16
17The alloc_sem is a per-filesystem mutex, used primarily to ensure
18contiguous allocation of space on the medium. It is automatically
19obtained during space allocations (jffs2_reserve_space()) and freed
20upon write completion (jffs2_complete_reservation()). Note that
21the garbage collector will obtain this right at the beginning of
22jffs2_garbage_collect_pass() and release it at the end, thereby
23preventing any other write activity on the file system during a
24garbage collect pass.
25
26When writing new nodes, the alloc_sem must be held until the new nodes
27have been properly linked into the data structures for the inode to
28which they belong. This is for the benefit of NAND flash - adding new
29nodes to an inode may obsolete old ones, and by holding the alloc_sem
30until this happens we ensure that any data in the write-buffer at the
31time this happens are part of the new node, not just something that
32was written afterwards. Hence, we can ensure the newly-obsoleted nodes
33don't actually get erased until the write-buffer has been flushed to
34the medium.
35
36With the introduction of NAND flash support and the write-buffer,
37the alloc_sem is also used to protect the wbuf-related members of the
38jffs2_sb_info structure. Atomically reading the wbuf_len member to see
39if the wbuf is currently holding any data is permitted, though.
40
41Ordering constraints: See f->sem.
42
43
44    File Mutex f->sem
45    ---------------------
46
47This is the JFFS2-internal equivalent of the inode mutex i->i_sem.
48It protects the contents of the jffs2_inode_info private inode data,
49including the linked list of node fragments (but see the notes below on
50erase_completion_lock), etc.
51
52The reason that the i_sem itself isn't used for this purpose is to
53avoid deadlocks with garbage collection -- the VFS will lock the i_sem
54before calling a function which may need to allocate space. The
55allocation may trigger garbage-collection, which may need to move a
56node belonging to the inode which was locked in the first place by the
57VFS. If the garbage collection code were to attempt to lock the i_sem
58of the inode from which it's garbage-collecting a physical node, this
59lead to deadlock, unless we played games with unlocking the i_sem
60before calling the space allocation functions.
61
62Instead of playing such games, we just have an extra internal
63mutex, which is obtained by the garbage collection code and also
64by the normal file system code _after_ allocation of space.
65
66Ordering constraints:
67
68    1. Never attempt to allocate space or lock alloc_sem with
69       any f->sem held.
70    2. Never attempt to lock two file mutexes in one thread.
71       No ordering rules have been made for doing so.
72
73
74    erase_completion_lock spinlock
75    ------------------------------
76
77This is used to serialise access to the eraseblock lists, to the
78per-eraseblock lists of physical jffs2_raw_node_ref structures, and
79(NB) the per-inode list of physical nodes. The latter is a special
80case - see below.
81
82As the MTD API no longer permits erase-completion callback functions
83to be called from bottom-half (timer) context (on the basis that nobody
84ever actually implemented such a thing), it's now sufficient to use
85a simple spin_lock() rather than spin_lock_bh().
86
87Note that the per-inode list of physical nodes (f->nodes) is a special
88case. Any changes to _valid_ nodes (i.e. ->flash_offset & 1 == 0) in
89the list are protected by the file mutex f->sem. But the erase code
90may remove _obsolete_ nodes from the list while holding only the
91erase_completion_lock. So you can walk the list only while holding the
92erase_completion_lock, and can drop the lock temporarily mid-walk as
93long as the pointer you're holding is to a _valid_ node, not an
94obsolete one.
95
96The erase_completion_lock is also used to protect the c->gc_task
97pointer when the garbage collection thread exits. The code to kill the
98GC thread locks it, sends the signal, then unlocks it - while the GC
99thread itself locks it, zeroes c->gc_task, then unlocks on the exit path.
100
101
102    inocache_lock spinlock
103    ----------------------
104
105This spinlock protects the hashed list (c->inocache_list) of the
106in-core jffs2_inode_cache objects (each inode in JFFS2 has the
107correspondent jffs2_inode_cache object). So, the inocache_lock
108has to be locked while walking the c->inocache_list hash buckets.
109
110This spinlock also covers allocation of new inode numbers, which is
111currently just '++->highest_ino++', but might one day get more complicated
112if we need to deal with wrapping after 4 milliard inode numbers are used.
113
114Note, the f->sem guarantees that the correspondent jffs2_inode_cache
115will not be removed. So, it is allowed to access it without locking
116the inocache_lock spinlock.
117
118Ordering constraints:
119
120    If both erase_completion_lock and inocache_lock are needed, the
121    c->erase_completion has to be acquired first.
122
123
124    erase_free_sem
125    --------------
126
127This mutex is only used by the erase code which frees obsolete node
128references and the jffs2_garbage_collect_deletion_dirent() function.
129The latter function on NAND flash must read _obsolete_ nodes to
130determine whether the 'deletion dirent' under consideration can be
131discarded or whether it is still required to show that an inode has
132been unlinked. Because reading from the flash may sleep, the
133erase_completion_lock cannot be held, so an alternative, more
134heavyweight lock was required to prevent the erase code from freeing
135the jffs2_raw_node_ref structures in question while the garbage
136collection code is looking at them.
137
138Suggestions for alternative solutions to this problem would be welcomed.
139
140
141    wbuf_sem
142    --------
143
144This read/write semaphore protects against concurrent access to the
145write-behind buffer ('wbuf') used for flash chips where we must write
146in blocks. It protects both the contents of the wbuf and the metadata
147which indicates which flash region (if any) is currently covered by
148the buffer.
149
150Ordering constraints:
151    Lock wbuf_sem last, after the alloc_sem or and f->sem.
152
153
154    c->xattr_sem
155    ------------
156
157This read/write semaphore protects against concurrent access to the
158xattr related objects which include stuff in superblock and ic->xref.
159In read-only path, write-semaphore is too much exclusion. It's enough
160by read-semaphore. But you must hold write-semaphore when updating,
161creating or deleting any xattr related object.
162
163Once xattr_sem released, there would be no assurance for the existence
164of those objects. Thus, a series of processes is often required to retry,
165when updating such a object is necessary under holding read semaphore.
166For example, do_jffs2_getxattr() holds read-semaphore to scan xref and
167xdatum at first. But it retries this process with holding write-semaphore
168after release read-semaphore, if it's necessary to load name/value pair
169from medium.
170
171Ordering constraints:
172    Lock xattr_sem last, after the alloc_sem.
173

Archive Download this file



interactive